In this project the specific objectives are to define the unique epidemiologic, clinical, virologic, and immunologic features of HIV infection in developing countries to determine the viral kinetics associated with perinatal and heterosexual transmission, and to characterize the molecular strains of HIV throughout the world for infectiousness and the immunologic response to cross-clade vaccines. In collaborative studies we have established cohorts of high-risk individuals in Uganda, Congo, South Africa, India, China and the U.S. We have extended our previous investigations in the Rakai district of Uganda by examining the biological factors associated with HIV transmission among discordant couples. Among 235 HIV-1-discordant couples with monogamous uninfected partners, the rate of transmission was 0.0082/coital act within an interval of approximately 2.5 months after index seroconversion. This probability of transmission decreased to 0.0015 per coital act 6-15 months after seroconversion and then stabilized at an average rate of 0.0007 per act among HIV-prevalent index partners. The probability of transmission increased significantly to 0.0028/act in the period 25 to 6 months before index partner development of AIDS. In multivariate adjusted models, early and late stage infection, higher viral load, genital ulcer disease and younger age in the index HIV-infected partner were associated with higher rates of transmission per coital act. HIV acquisition was significantly associated with HSV-2 seropositivity with an odds ratio of 2.6. In addition, at five months and 15 months post-seroconversion HIV viral load was significantly higher in HSV-2 seropositive compared to HSV-2 seronegative individuals. This interaction of HSV-2 and HIV with subsequent increased viral loads and increased HIV and HSV shedding from active ulcers in HIV-infected people supports the needs for HSV-2 treatment and prophylaxis in infected individuals. To document viral transmission among partners, molecular sequencing of HIV was performed in samples from 419 HIV-infected individuals, including 84 transmission pairs. Of the 84 transmission pairs, 86% were infected by their sexual partner as indicated by sequence matches. For individuals who reported a monogamous relationship, viral sequences matched in 92% compared to non-monogamous relationships where transmission linkage occurred in 68%. HIV sequence subtyping was performed in a case-controlled study design of 180 retrospectively identified monogamous couples in Rakai. 70% were subtype D, 9% were subtype A, and 21% were recombinant A/D. At viral loads < 104 copies/ml, subtype D was less transmissible than subtype A or recombinant A/D, although at viral loads at > 104 copies/ml, there was no difference in risk of transmission observed. 62 viral strains were also phenotyped for co-receptor tropism and an increased frequency for CXCR4 tropism was observed for subtype D. This increased expression of CXCR4 in subtype D may partially explain the decrease in efficiency of transmission at low viral loads for this subtype compared to subtype A or recombinant A/D. Additional subtyping of the Rakai cohort was performed to determine the regional and temporal distribution of HIV-1 subtypes. The distribution of D, A, and recombinant subtypes was 67%, 15%, and 17% in 1994 and 67%, 12%, and 21% in 1997. Subtype D was most prevalent in the two urban areas, subtype C was prevalent in only one region populated by transitory government workers and the region with the highest percentage of recombinant virus (32%) was the one closest to the trans-African highway. These data suggest that there was little change in distribution of viral subtypes in Rakai over a four-year period, but there was significant diversity in viral subtype based on locality suggesting microepidemics that are focal in nature. In Kampala, Uganda, we measured the impact of antiretroviral therapy among 137 consecutive patients treated with ART. At enrollment 8% were WHO clinical stage II, 25% stage III, and 67% were stage IV. The mean CD4 was 50, which subsequently rose with therapy to 174 cells/mm3. 78% of participants being treated with their first ARV regimen achieved viral suppression to <400 copies/ml. At follow-up 12.9% had discontinued treatment usually due to initiation of TB therapy. Participants reported adherence rates of 90%. 85% reported a marked improvement in their health status, 94% reported better performance at home or work, 98% reported feeling stronger, and 92% reported increased energy. 93% understood that missing doses encouraged viral resistance. HIV-infected individuals in resource-limited settings derived significant benefit from treatment with ART and the health beliefs and behaviors reported in this population on ART are associated with positive health outcomes. The significance of these studies is that they provide important epidemiologic, clinical, virologic, and immunologic knowledge of HIV infection in developing countries, which can be utilized for monitoring future trends of the epidemic and developing behavioral and biological interventions to prevent further transmission.